Continuous casting of steel is carried out while molten metal in a tundish is supplied into a mold of continuous casting equipment via an immersion nozzle. The molten steel is discharged from an outlet port that is formed in a lower end portion of the immersion nozzle, into the mold, is cooled in the mold, and is withdrawn from a mold outlet in the state where a thickness of a solidified shell enough to prevent breakout is ensured. The solidified shell is completely solidified by secondary cooling with spray during the process of withdrawn, and is cut, to be a cast steel.
As a technique of improving the cleanliness of a cast steel, for example, Patent Literature 1 discloses that electromagnetic stirrers are oppositely arranged in the vicinity of a meniscus in long sides of a mold, so that a swirl flow is generated on the surface of molten steel in the mold; the cleaning effect of this swirl flow checks the phenomenon of adhesion of inclusions and bubbles to the surface of the mold, which is a main cause of defects in a cast steel. Patent Literature 2 discloses that an electromagnetic brake is operated on an outlet flow that is discharged from an outlet port of an immersion nozzle, so as to hold down the descending speed of molten steel, to have time for inclusions in the molten steel to float up.
In the technique of the above Patent Literature 1, an electromagnetic brake does not work on an outlet flow discharged from an outlet port of an immersion nozzle. Thus, the descending speed of the outlet flow is not held down. Therefore, bubbles and inclusions such as alumina remaining in molten steel do not float up or are not removed enough, and they infiltrate into a deep portion of the cast steel, to be a cause of internal defects, which is problematic. This problem can be avoided by operating the electromagnetic brake on the outlet flow as the above described Patent Literature 2.
When an electromagnetic brake is operated on an outlet flow, as shown in FIG. 3 (a front cross-sectional view of a mold) and 4 (a side cross-sectional view of the mold), an upward flow along an immersion nozzle 2 is generated. This upward flow turns around near the surface of molten steel, to be a downward flow. Here, specifically, a distance (D0) between long side surfaces of the mold for manufacturing a thin cast steel is short. Therefore, inclusions and bubbles carried by the downward flow are easy to be in contact with a solidified shell 8 that is formed on long side walls 3a and 3b composing long sides of the mold, and caught here, to be a main cause of surface defects, which is a new problem.